During operation of a nuclear power reactor, impurities and products of the reactor coolant are deposited on nuclear fuel assemblies. These deposits can impact operation and maintenance of nuclear power plants in a number of ways; for example, (a) their neutronic properties can adversely affect the nuclear performance of the reactor; (b) their thermal resistance can cause elevated surface temperature on the fuel rods that may lead to material failure in the rod; (c) their radioactive decay results in work radiation exposure when they are redistributed throughout the reactor coolant system, in particular during power transients; (d) they complicate thorough inspection of irradiated nuclear fuel assemblies by both visual and eddy current methods; (e) deposits released from fuel rods tend to reduce visibility in the spent fuel pool, significantly delaying other work in the fuel pool during refueling outages; (f) once reloaded into the reactor on assemblies that will be irradiated a second or third time, they form an inventory of material that can be redistributed onto new fuel assemblies in a detrimental manner. Currently, methods to efficiently and cost-effectively remove such deposits from irradiated nuclear fuel assemblies are lacking other than slow, manual techniques.
Recently, axial offset anomaly (AOA) has been reported in pressurized water reactors (PWRs). AOA is a phenomenon in which deposits form on the fuel rod cladding due to the combination of local thermal-hydraulic conditions and primary-side fluid impurities characteristic of the reactor and the primary system. These deposits act as a poison to the nuclear reaction and cause an abnormal power distribution along the axis of the core, reducing available margin under certain operating conditions. AOA has forced some power plants to reduce the reactor power level for extended periods.
The problem of AOA has necessitated the development of an efficient, cost-effective mechanism for removing PWR fuel deposits. Such a mechanism is also desirable to reduce total deposit inventory to lower dose rates for plant personnel, to improve fuel inspectability, to prepare fuel for long-term dry storage, and to facilitate the collection of crud samples for analysis.
Several approaches have been proposed to remove PWR fuel deposits. One method is to chemically clean assemblies in situ in the reactor, or after being removed to a separate cleaning cell. There are several problems with this approach, including cost, potential for corrosion by the cleaning chemicals, and the difficulty of disposing of the resultant highly contaminated chemicals. Perhaps the greatest shortcoming of this chemical approach is that it is time consuming, requiring several hours to clean a single fuel assembly.
Another approach being pursued is circulation of ice chips in a cleaning cell where the flow of ice past the fuel rods would gently remove deposits. There are concerns with this approach, including cleaning effectiveness, the difficulty of driving ice chips through certain fuel support structures, the need to create large volumes of ice chips, the effect of low temperatures on the structural integrity of the fuel rods, and the dilution of Boron in the spent fuel pool.
In the past, individual fuel rods and fuel channels have been cleaned by conventional ultrasonics during the manufacturing process. However, conventional ultrasonics would not be very effective in cleaning large bundles of fuel rods in irradiated fuel assemblies due to the low power density per unit volume that can be produced. Furthermore, the conventional ultrasonic cleaning transducers are large and therefore difficult to implement in a typical plant fuel pool.
In view of the foregoing, it would be highly desirable to provide a time-efficient, effective, low-cost technique to remove deposits from irradiated nuclear fuel assemblies.